1. Field of the Invention
The present invention relates to image forming apparatuses, and more particularly, to an image forming apparatus forming an image by transferring selectively charged toner to a recording member such as a sheet-like member.
2. Description of the Related Art
In a conventional image forming apparatus forming an electric image signal as a visual image, image data was optically drawn on a recording medium including a photosensitive layer having electro-optic characteristics such as the surface of a photoreceptor using particulate toner serving as a coloring agent to form an electrostatic latent image (electrostatic pattern). The toner was attached to the electrostatic latent image to form the visual image. The visualized toner image was transferred to a sheet-like member such as a papersheet to form a desired image.
As the above described image forming apparatus employing electrophotography, a laser printer, an LED printer, and the like are well known.
In order to form an image on a photoreceptor, such a printer employing electrophotography requires a plurality of units constituting an image forming system around the photoreceptor. The printer requires a charger, an optical system (for example, laser irradiating means), a developing device, a transferring device, a cleaning device, a discharger, and the like. Therefore, the printer is under a restriction on reduction in size.
On the other hand, in order to form an image using toner without employing the above described electrophotography, toner held on a toner holder is selectively charged, and the selectively charged toner is transferred to a papersheet which is sequentially fed to form a desired image. An apparatus for forming an image in this manner is disclosed in Japanese Patent Laying-Open No. 60-149057, for example.
Referring to FIG. 10, the principle of image formation disclosed in this publication will be described. The image forming apparatus in FIG. 10 includes a toner hopper 101 housing one-component toner, a toner holder 102 holding the toner, a doctor blade 103 regulating the amount of toner held on the surface of toner holder 102, an electric charge injecting element 104 selectively charging the toner held by toner holder 102, for example, selectively injecting electric change to the toner held by toner holder 102, a transfer roller 106 for transferring the selectively charged toner to a recording sheet 105, and a control slit 107 disposed between transfer roller 106 and toner holder 102 for controlling transfer of the selectively charged toner.
Image forming operation based on the principle will be described. The toner in toner hopper 101 is held by toner holder 102. The amount of toner is regulated by doctor blade 103 to form a toner layer having a predetermined thickness on toner holder 102. By being supplied with a voltage having a specific polarity, doctor blade 103 is charged to the polarity. The toner held by toner holder 102 is selectively charged by electric charge having the opposite polarity through electric charge injecting element 104 according to a desired image. As a result, an electric latent image (for example, electrostatic latent image) is formed on the toner layer held on the surface of toner holder 102. When the toner forming the latent image reaches the position of transfer roller 106, the toner is influenced by the voltage applied to transfer roller 106 to be attracted electrostatically and to be transferred to sheet 105. When the toner passes through control slit 107, control slit 107 prevents repulsion among pieces of the toner to transfer the toner onto sheet 105 by narrowing down the transfer width of the toner. Since the toner transferred onto sheet 105 as described above has not been fixed on sheet 105, the toner is fixed in the following step, enabling formation of a desired image on the sheet.
The image forming apparatus disclosed in Japanese Patent Laying-Open No. 60-149047 does not require a photoreceptor, unlike the image forming apparatus employing electrophotography. This image forming apparatus of Japanese Patent Laying-Open No. 60-149047 selectively charges toner held by the toner holder, and transfers the selectively charged toner to a sheet. Therefore, a charger, an image exposing area, a discharger, and a cleaning device are not required, resulting in reduction in size and cost.
However, the above described image forming apparatus regulates by the doctor blade the thickness of the toner layer held by the toner holder, and selectively charges the toner layer formed by the electric charge injecting element, resulting in increase in the number of components.
Further, in order to inject electric charge in the toner using the electric charge injecting element, the conductive toner must be used in the above described image forming apparatus. More specifically, if the toner is not conductive, electric charge is not injected. Since the conductive toner must contain a substance other than a coloring agent in order to increase the conductivity, it becomes difficult to prepare color toner. Therefore, it is difficult to obtain three-color toner for full color image formation.
If insulative toner is used, three-color toner for full color image formation can be obtained. However, if the insulative toner is used in the image forming apparatus injecting electric charge to the toner as described above, desired electric charge cannot be injected in the toner. Further, when the selectively charged conductive toner is transferred onto a sheet, the conductive toner is less likely to be transferred to the sheet if the sheet is conductive. Therefore, an insulated sheet having high resistance is required, and the image forming apparatus is further under a restriction on the sheet.
Further, when the toner held on the toner holder and later selectively charged is transferred onto the sheet, a transfer roller is used. A toner transfer area is determined by the curvature of the transfer roller. If the transfer area is large, the toner starts to be transferred gradually before the sheet completely reaches the transfer area, causing degradation of resolution.